Eilenberg-Steenrod homology theories #
In this file we introduce the Eilenberg-Steenrod axioms for homology theories.
The data for a homology theory is bundled in a structure HomologyPretheory consisting of functors
Hₚ i : TopPair ⥤ C and H i : TopCat ⥤ C which represent the ith relative and regular homology,
respectively, (indexed by a ComplexShape) and a proof that they agree on TopCat. They also
require boundary morphisms δ i j : Hₚ i ⟶ proj₂ ⋙ H j for the long exact sequence of
topological pairs. These are nonzero only if c.Rel i j.
We introduce a type class for each axiom:
IsHomotopyInvariant: Homotopic maps induce the same map in homology.HasExcisionIso: For a sufficiently nice subsetU ⊆ A, the inclusion(X \ U, A \ U) → (X, A)induces an isomorphism in homology.IsAdditive: Homology preserves coproducts.HasPairSequence: For a topological pair(X, A), the sequence⋯ ⟶ H i X ⟶ H i (X, A) ⟶ H (i - 1) A ⟶ H (i - 1) X ⟶ H (i - 1) (X, A) ⟶ ⋯is exact.HasDimensionAxiom: AHomologyPretheoryonComplexShape.down ℕ : ComplexShape ℕhas the dimension axiom, if homology is zero for positive indices.
In addition, there are bundled type classes IsExtraordinaryEilenbergSteenrod with the
homotopy-invariance, excision, additivity, and pair sequence axioms and IsEilenbergSteenrod which
extends the former by the dimension axiom.
A HomologyPretheory is the data of an Eilenberg-Steenrod homology theory.
- Hₚ (i : ι) : CategoryTheory.Functor TopPair C
The relative homology functor of a
HomologyPretheory. - H (i : ι) : CategoryTheory.Functor TopCat C
The regular homology functor of a
HomologyPretheory. The boundary natural transformation of a
HomologyPretheory.The boundary map is only nonzero if
c.Rel i j.
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A morphism in the category HomologyPretheory.
The natural transformation of relative homology functors in a morphism of
HomologyPretheorys.The natural transformation of homology functors in a morphism of
HomologyPretheorys.- iso_comm (i : ι) : CategoryTheory.CategoryStruct.comp (HP.iso i).hom (incl.whiskerLeft (self.homₚ i)) = CategoryTheory.CategoryStruct.comp (self.hom i) (HP'.iso i).hom
homₚandhomneed to be compatible withHomologyPretheory.iso. - w (i j : ι) : CategoryTheory.CategoryStruct.comp (HP.δ i j) (proj₂.whiskerLeft (self.hom j)) = CategoryTheory.CategoryStruct.comp (self.homₚ i) (HP'.δ i j)
homₚneeds to be compatible with the boundary maps.
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homₚ and hom need to be compatible with HomologyPretheory.iso.
homₚ needs to be compatible with the boundary maps.
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The map H i X.fst ⟶ Hₚ i X induced by the inclusion.
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- TopPair.HomologyPretheory.hFstToHₚ i X = CategoryTheory.CategoryStruct.comp ((HP.iso i).hom.app TopPair.fst) ((HP.Hₚ i).map X.j)
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The forgetful functor that sends a HomologyPretheory to it's relative homology functor Hₚ.
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The isomorphism of relative homology that is part of an isomorphism of HomologyPretheorys.
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The forgetful functor that sends a HomologyPretheory to it's homology functor H.
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The isomorphism of absolute homology that is part of an isomorphism of HomologyPretheorys.
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- TopPair.HomologyPretheory.hIsoOfIso e i = HP.iso i ≪≫ TopPair.incl.isoWhiskerLeft ((TopPair.HomologyPretheory.hₚFunctor i).mapIso e) ≪≫ (HP'.iso i).symm
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A HomologyPretheory is homotopy-invariant if its homology functor Hₚ takes homotopic maps to
the same map in homology
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An abbreviation for HomologyPretheory.IsHomotopyInvariant as ObjectProperty.
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A HomologyPretheory has the excision-isomorphism, if cutting out a sufficiently nice subspace
U from a space X yields an isomorphism Hₚ i X ≅ Hₚ i (X \ U).
- isIso_of_isExcisive ⦃X V : TopPair⦄ (g : V ⟶ X) [IsExcisive g] (i : ι) : CategoryTheory.IsIso ((HP.Hₚ i).map g)
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An abbreviation for HomologyPretheory.HasExcisionIso as ObjectProperty.
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A HomologyPretheory is additive if its homology functor preserves coproducts.
- preserves_coproducts (J : Type u) (i : ι) : CategoryTheory.Limits.PreservesColimitsOfShape (CategoryTheory.Discrete J) (HP.H i)
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An abbreviation for HomologyPretheory.IsAdditive as ObjectProperty.
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This imposes that a HomologyPretheory has the long exact sequence of topological pairs
⋯ ⟶ H (c.next i) X.fst ⟶ Hₚ (c.next i) X) ⟶ H i X.snd ⟶ H i X.fst ⟶ ⋯.
- epi_map_of_not_rel (X : TopPair) (i : ι) (hi : ∀ (j : ι), ¬c.Rel i j) : CategoryTheory.Epi ((HP.Hₚ i).map X.j)
The boundary map out of homology with final index (i.e. an index with no successors in the
ComplexShape) is an epi.
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An abbreviation for HomologyPretheory.HasPairSequence as ObjectProperty.
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An extraordinary Eilenberg-Steenrod homology theory requires the homotopy, excision, additivity, and exactness axioms.
- isHomotopyInvariant : HP.IsHomotopyInvariant
Invariance of an extraordinary Eilenberg-Steenrod homology theory on homotopic maps.
- hasExcisionIso : HP.HasExcisionIso
Excision axiom of an extraordinary Eilenberg-Steenrod homology theory.
- isAdditive : HP.IsAdditive
An extraordinary Eilenberg-Steenrod homology functor preserves coproducts.
- hasPairSequence : HP.HasPairSequence
The long exact sequence of topological pairs in an extraordinary Eilenberg-Steenrod homology theory.
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An abbreviation for HomologyPretheory.IsExtraordinaryEilenbergSteenrod as ObjectProperty.
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A HomologyPretheory on ComplexShape.down ℕ has the dimension axiom if it is trivial on the
terminal space for n > 0.
- isZero_pUnit_of_gt_zero (n : ℕ) : n ≠ 0 → CategoryTheory.Limits.IsZero ((HP.H n).obj ↧PUnit.{u + 1})
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An abbreviation for HomologyPretheory.HasDimensionAxiom as ObjectProperty.
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An Eilenberg-Steenrod homology theory is an extraordinary Eilenberg-Steenrod homology theory which additionally satisfies the dimension axiom.
- isAdditive : HP.IsAdditive
- hasDimensionAxiom : HP.HasDimensionAxiom
An Eilenberg-Steenrod homology theory is trivial on the terminal space for
n > 0.
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An abbreviation for HomologyPretheory.IsEilenbergSteenrod as ObjectProperty.